Vertebrates are constantly threatened by the invasion of microorganisms and have evolved mechanisms of immune defense to eliminate infective pathogens. In mammals, this immune system comprises two branches; innate immunity and adaptive immunity. The innate immune system is the first line of defense which is initiated by Pattern Recognition Receptors (PRRs) which detect ligands from the pathogens as well as damage associated molecular patterns (Takeuchi O. et al, Cell, 2010: 140, 805-820). A growing number of these receptors have been identified including Toll-like receptors (TLRs), C-type lectin receptors, retinoic acid inducible gene I (RIG-I)-like receptors and NOD-like receptors (NLRs) and also double stranded DNA sensors. Activation of PRRs leads to up-regulation of genes involved in the inflammatory response including type 1 interferons, pro-inflammatory cytokines and chemokines which suppress pathogen replication and facilitate adaptive immunity.
The adaptor protein STING (Stimulator of Interferon Genes), also known as TMEM 173, MPYS, MITA and ERIS, has been identified as a central signalling molecule in the innate immune response to cytosolic nucleic acids (Ishikawa H and Barber G N, Nature, 2008: 455, 674-678; WO2013/1666000). Activation of STING results in up-regulation of IRF3 and NFκB pathways leading to induction of Interferon-β and other cytokines. STING is critical for responses to cytosolic DNA of pathogen or host origin, and of unusual nucleic acids called Cyclic Dinucleotides (CDNs)
CDNs were first identified as bacterial secondary messengers responsible for controlling numerous responses in the prokaryotic cell. Bacterial CDNs, such as c-di-GMP are symmetrical molecules characterised by two 3′,5′ phophodiester linkages.

Direct activation of STING by bacterial CDNs has recently been confirmed through X-ray crystallography (Burdette D L and Vance R E, Nature Immunology, 2013: 14, 19-26). Bacterial CDNs and their analogues have consequently attracted interest as potential vaccine adjuvants (Libanova R. et al, Microbial Biotechnology 2012: 5, 168-176; WO2007/054279, WO2005/087238).
More recently, the response to cytosolic DNA has been elucidated and shown to involve generation, by an enzyme called cyclic GMP-AMP synthase (cGAS, previously known as C6orf150 or MB21D1), of a novel mammalian CDN signalling molecule identified as cGAMP, which then activates STING. Unlike bacterial CDNs, cGAMP is an unsymmetrical molecule characterised by it's mixed 2′,5′ and 3′,5′ phosphodiester linkages. (Gao P et al, Cell, 2013: 153, 1-14). Interaction of cGAMP with STING has also been demonstrated by X-ray crystallography (Cai X et al, Molecular Cell, 2014: 54, 289-296).
Interferon was first described as a substance which could protect cells from viral infection (Isaacs & Lindemann, J. Virus Interference. Proc. R. Soc. Lon. Ser. B. Biol. Sci. 1957: 147, 258-267). In man, the type I interferons are a family of related proteins encoded by genes on chromosome 9 and encoding at least 13 isoforms of interferon alpha (IFNα) and one isoform of interferon beta (IFNβ). Recombinant IFNα was the first approved biological therapeutic and has become an important therapy in viral infections and in cancer. As well as direct antiviral activity on cells, interferons are known to be potent modulators of the immune response, acting on cells of the immune system.
Administration of a small molecule compound which could stimulate the innate immune response, including the activation of type I interferons and other cytokines, could become an important strategy for the treatment or prevention of human diseases including viral infections. This type of immunomodulatory strategy has the potential to identify compounds which may be useful not only in infectious diseases but also in cancer (Zitvogel, L., et al., Nature Reviews Immunology, 2015 15(7), p 405-414), allergic diseases (Moisan J. et al, Am. J. Physiol. Lung Cell Mol. Physiol., 2006: 290, L987-995), other inflammatory conditions such as irritable bowel disease (Rakoff-Nahoum S., Cell., 2004, 23, 118(2): 229-41), and as vaccine adjuvants (Persing et al. Trends Microbiol. 2002: 10(10 Suppl), S32-7 and Dubensky et al., Therapeutic Advances in Vaccines, published on line Sep. 5, 2013).
Allergic diseases are associated with a Th2-biased immune-response to allergens. Th2 responses are associated with raised levels of IgE, which, via its effects on mast cells, promotes a hypersensitivity to allergens, resulting in the symptoms seen, for example, in allergic rhinitis and asthma. In healthy individuals the immune-response to allergens is more balanced with a mixed Th2/Th1 and regulatory T cell response. Induction of Type 1 interferons have been shown to result in reduction of Th2-type cytokines in the local environment and promote Th1/Treg responses. In this context, induction of type 1 interferons by, for example, activation of STING, may offer benefit in treatment of allergic diseases such as asthma and allergic rhinitis (Huber J. P. et al J Immunol 2010: 185, 813-817).
In contrast, increased and prolonged type I IFN production is associated with a variety of chronic infections, including Mycobacteria (Collins et al, CHM 2015; Wassermann et al., CHM 2015; Watson et al., CHM 2015), Franciscella (Storek et al., JI 2015; Jin et al., JI 2011), Chlamydia (Prantner et al., JI 2010; Barker et al., Mbio 2013; Zhang et al., JI 2014), Plasmodium (Sharma et al., Immunity 2011) and HIV (Herzner et al., Nat Immunol 2015; Nissen et al., Clin Exp Immunol 2014; Gao et al., Science 2013; Lahaye et al, Science 2013;) (reviewed in Stiffer and Feng, J I 2014). Similarly, excess type I interferon production is found among patients with complex forms of autoimmune disease. Genetic evidence in humans and support from studies in animal models support the hypothesis that inhibition of STING results in reduced type I interferon that drives autoimmune disease (Crow Y J, et al., Nat. Genet. 2006; 38917-920, Stetson D B, et al., Cell 2008; 134; 587-598). Therefore, inhibitors of STING provide a treatment to patients with chronic type I interferon and proinflammatory cytokine production associated with infections or complex autoimmune diseases. Allergic diseases are associated with a Th2-biased immune-response to allergens.
Compounds that bind to STING and act as agonist have been shown to induce type 1 interferons and other cytokines on incubation with human PBMCs. Compounds which induce human interferons may be useful in the treatment of various diseases, for example the treatment of allergic diseases and other inflammatory diseases for example allergic rhinitis and asthma, the treatment of infectious diseases, pre-cancerous syndromes and cancer, and may also be useful as immugenic composition or vaccine adjuvants. Compounds that bind to STING may act as antagonists and could be useful in the treatment, for example of autoimmune diseases.
It is envisaged that targeting STING with activation or inhibiting agents may be a promising approach for treating diseases in which modulation for the type 1 IFN pathway is beneficial, including inflammatory, allergic and autoimmune diseases, infectious diseases, cancer, pre-cancerous syndromes and as immugenic composition or vaccine adjuvants.
International Patent Applications WO2014/093936, WO2014/189805, WO2013/185052, U.S.2014/0341976, WO 2015/077354, PCT/EP2015/062281 and GB 1501462.4 disclose certain cyclic di-nucleotides and their use in inducing an immune response.
It is an object of the invention to provide further cyclic di-nucleotides, suitably for the treatment of cancer.